Pepsinogen C (PGC) and mucin1 (MUC1) are important physiologically functional gastric proteins; Mucin2 (MUC2) is an “ectopic” functional protein in intestinal metaplasia of gastric mucosa. We analyzed the co-expression of the these three proteins in a dynamic gastric disease sequence as well as different histological types of gastric cancer in order to explore the co-expression-based molecular phenotypes of gastric cancer as well as its precancerous disease and correlation between the co-expression pattern and different gastric diseases.
As we know, PGC, MUC1, MUC2, the three proteins had solely important diagnostic role for the gastric disease. But the co-expression as well as their molecular phenotype had not been reported until now, while in distinction between different gastric diseases. In fact, the co-expression of combined proteins suggesting molecular phenotype like MUC2 and CD10 had been reported previously. Wakatsuki, K et al. and Hasuo, T et al. divided GC into the gastric phenotype (G-type) and intestinal phenotype (I-type) according to MUC5AC, MUC6, MUC2, and CD10 [30, 31], while other scholars divided G-type and I-type according to MUC2 with other proteins [20, 32]. But the significance of these studies was all based on the consideration that different phenotypes had different patterns progressing to GC. We aimed to investigate the co-expression of PGC, MUC1 and MUC2 which might also give a clue for the understanding of the progression of gastric diseases.
There were seven types of the co-expression for the studied three proteins in this study. In SG group it always showed the phenotype of PGC+/MUC1+/MUC2- which means PGC and MUC1 were both positive and MUC2 was negative. We also analyzed the correlations between the three proteins and the SG-AG-GC sequence and found that PGC and MUC1 both had negative correlation and only MUC2 had positive correlation with the disease sequence, which means PGC and MUC1 had similar distribution tendency while PGC had a larger correlation coefficient than MUC1 (r = −0.770 and r = −0.210, respectively). In AG group the co-expression of these three proteins showed two phenotypes (PGC+/MUC1+/MUC2+ and PGC-/MUC1+/MUC2+) and the main difference between them was whether PGC protein was expressed. PGC is a signal of gradually maturing of digestive function, a differentiation product of the digestive enzyme pepsin C, and it was reported that PGC gradually decreased in the SG–AG-GC sequence . In our study, PGC was nearly 50% positive and 50% negative in the AG group. In the dynamic change of SG to AG, part of samples appeared PGC negative while part of samples remained positive. Among the 50% positive samples, there still were 22.8% of the cases (13/57, Table 2) which were weak positive (scored 0.1-0.9). Why part of samples appeared negative while some still remained positive? There may be two hypotheses to explain: first, it may be associated with the degree of glandular atrophy; second, the genetic variability of human PGC between individuals may contribute to the different expression of PGC in AG group. Although the latter was just a hypothesis, other protein like hypoxic marker carbonic anhydrase (CA) IX had been reported that genetic methylation status was contribute to the different expression of CA IX in GC group . These hypotheses need to be investigated in the future. In GC group the co-expression of these three proteins appeared variable, but we found an interesting phenomenon. When analyzing different histological GC groups, we found the phenotype of PGC-/MUC1-/MUC2+ all distributed in the group of mucinous adenocarcinoma or signet ring cell carcinoma (MA or SRCC, 100%, 6/6). In the clinical pathological diagnosis, the histological mucinous adenocarcinoma and signet ring cell carcinoma were always hard to distinguish with tubular adenocarcinoma accompanied by mucinous secretion or signet ring cell scattered distribution. In our study, the phenotype of PGC-/MUC1-/MUC2+ all distributed in the group of MA or SRCC rather than other groups, which suggested that the histological worse differentiation MA or SRCC lost PGC and MUC1 which are biomarkers of mature differentiation. We found MUC1 positive in tubular adenocarcinoma. Even though it accompanied mucinous secretion or signet ring cell scattered distribution, it suggested a better differentiation so that biomarkers of mature differentiation like MUC1 and PGC could appear. The phenotype of PGC-/MUC1-/MUC2+ may be a predictive biomarker for diagnosing MA or SRCC or distinguishing from tubular adenocarcinoma accompanied by mucinous secretion or signet ring cell scattered distribution. Choi and his colleagues found mucinous adenocarcinoma always showed MUC1- and MUC2+ in a study of 133 MA cases , which was consistent with the result of our study. Could MUC2 solely identify mucinous adenocarcinoma? Probably no because it was not specific since part of the MUC2+ phenotype of GC belongs to tubular adenocarcinoma. After adding MUC1 which was a biomarker of mature differentiation to limit, the identification of MA or SRCC turned out to be more sensitive.
It is fully aware that our study had some limitations. First, only gastroscopic biopsy specimens were adopted and limited clinical data (only age and gender) were available. Second, the sample of the cases was relatively small especially GC group and its subgroup analysis. Future larger sample study was required to validate our result. Third, precancerous diseases only included AG group while other types of precancerous diseases such as adenomas were not assessed because the sample size of other precancerous diseases like adenomas was too small to be a group. Fourth, the current study only discussed two mucins (i.e. MUC1 and MUC2) and one pepsinogen PGC without inclusion of other mucins like MUC4 which has been reported increased expression in different types of gastric cancer like adenocarcinoma and SRCC  or other gastric functional proteins like pepsinogen A  and trefoil factors family [37–40]. Further investigation including stomach-related proteins could give a profile of the progression and also biomarker of gastric diseases.